Introduction
Selective Laser Sintering (SLS) is an additive manufacturing method commonly used for rapid prototyping and low-volume production of functional parts.
During SLS, a laser beam is used to sinter powdered material, binding it together to form a solid structure. By scanning cross-sections generated from a 3D digital model of the required part, the laser selectively fuses pre-defined areas of a powder bed. Once each cross-section is scanned, a new material layer is applied on top. This process is replicated until the component part is complete.
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Generating powder layers is a precise process that demands a feedstock that the delivery system can reliably distribute. The material must be consistently deposited onto the fabrication bed without forming agglomerates or voids.
Intermittent flow or agglomerates within the bulk can lead to non-uniform deposition, negatively impacting the process's efficiency and the final product's properties. By identifying the powder properties that promote the formation of uniform, repeatable layers, new formulations can be optimized, and suitable raw materials can be selected without the substantial financial and time costs associated with running materials through a process to assess compatibility.
This method also helps reduce the likelihood of producing end products that are out of specification.
The Effect of Different Additives
Three samples of Polyoxymethylene (POM), including two with different additives (a pigment and a lubricant), were used in an SLS machine. The three formulations flowed differently from the storage hopper into the machine, leading to differences in the characteristics and quality of the product.
Traditional characterization techniques had been employed, but they failed to differentiate between the samples. Subsequently, the three formulations were analyzed using an FT4 Powder Rheometer®, which revealed clear and repeatable differences that explained the variations in in-process performance.
Test Results
Dynamic Testing: Basic Flowability Energy
The sample containing the flow additive exhibited a higher Basic Flowability Energy (BFE) than the other two samples, meaning it required more energy to move the FT4 blade through the powder bed. In this context, a high BFE indicates more efficient packing within the bulk, suggesting that the addition of the flow additive has produced a more free-flowing material.
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Bulk Testing: Permeability
The sample with the flow additive showed the highest Pressure Drop Across the Powder Bed at a low consolidating stress, suggesting lower permeability and indicating the more compact packing state of this freer-flowing substance. Although the pressure drop increased for all three samples under greater consolidation, the pure POM and the sample with pigment exhibited a much more significant change than the flow additive sample.
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Low sensitivity to changes in consolidation further indicates a more efficiently packed bulk, meaning there are fewer air voids for particles to occupy when subjected to external force. The sample containing pigment exhibited the greatest change in permeability, suggesting a larger volume of entrained air within the bulk, which is indicative of higher cohesivity.
Shear Cell Testing
Only minimal differences were observed between the samples in terms of measured shear stress values, suggesting that shear cell testing might not be the most suitable method for characterizing flow properties in the low-stress, dynamic processes common in SLS applications.
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Conclusion
The FT4 Powder Rheometer® has quantified clear and repeatable differences between three samples that are known to behave differently during processing. The results also illustrate that relying on a single technique, such as shear cell testing, may not be sufficient to fully characterize powder behavior across various stress and flow conditions.
Powder flowability is not an inherent property of the material but rather depends on its ability to flow in a desired manner within specific equipment. Successful processing requires that the powder and the process are well-matched, as a powder that performs well in one process may perform poorly in another.
This highlights the need for multiple characterization methodologies, which can be correlated with process rankings to create a design space of parameters corresponding to acceptable process behavior.
Instead of relying on a single measurement to describe behavior across all processes, the multivariate approach of the FT4 simulates a range of unit operations, enabling the direct investigation of a powder’s response to various process and environmental conditions.
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
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